Katherine Ashenburg, author of The Dirt on Clean: An Unsanitized History:
Oddly enough it was the Civil War that got Americans interested in being clean. The army’s initially derided Sanitary Commission, headed by the landscape architect Frederick Law Olmsted, had proved that simple soap and water could significantly reduce military mortality, and by the end of the war cleanliness was seen as patriotic, progressive and distinctively American. Good hygiene had other virtues: it was a way to mark status and civility in a country without an aristocracy, and it could "Americanize" the hundreds of thousands of Europeans who began arriving in the 1880s.
1898 ad by Proctor & Gamble
Those ideas drew 19th-century Americans to cleanliness, but it was advertising that kept them interested. Body soap first became widely available in the late 19th-century, around the birth of modern advertising. Since there was little to distinguish one soap from another, advertising and soap grew up together. All the gambits later used for a variety of products – celebrity testimonials, prizes, slogans and jingles – made their first appearances in ads for soap. By the end of the 19th-century, advertising’s biggest customers were patent medicines and soaps.(h/t The Dish)
